REVENUES - Disaggregation of Revenues (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of revenues
Total revenues	$ 180,492	$ 171,243	$ 352,220	$ 334,608
Point in time
Disaggregation of revenues
Total revenues	86,828		172,440
Over time
Disaggregation of revenues
Total revenues	93,664		179,780
United States
Disaggregation of revenues
Total revenues	135,285		259,788
International
Disaggregation of revenues
Total revenues	45,207		92,432
Commercial
Disaggregation of revenues
Total revenues	129,238		258,582
U.S. Government
Disaggregation of revenues
Total revenues	51,254		93,638
Aerospace & Defense
Disaggregation of revenues
Total revenues	95,124	79,478	172,635	145,702
Aerospace & Defense | Point in time
Disaggregation of revenues
Total revenues	46,610		82,897
Aerospace & Defense | Over time
Disaggregation of revenues
Total revenues	48,514		89,738
Aerospace & Defense | United States
Disaggregation of revenues
Total revenues	81,458		146,164
Aerospace & Defense | International
Disaggregation of revenues
Total revenues	13,666		26,471
Aerospace & Defense | Commercial
Disaggregation of revenues
Total revenues	51,550		93,417
Aerospace & Defense | U.S. Government
Disaggregation of revenues
Total revenues	43,574		79,218
Test
Disaggregation of revenues
Total revenues	41,600	42,875	82,983	84,161
Test | Point in time
Disaggregation of revenues
Total revenues	8,009		17,019
Test | Over time
Disaggregation of revenues
Total revenues	33,591		65,964
Test | United States
Disaggregation of revenues
Total revenues	25,121		49,959
Test | International
Disaggregation of revenues
Total revenues	16,479		33,024
Test | Commercial
Disaggregation of revenues
Total revenues	33,952		69,194
Test | U.S. Government
Disaggregation of revenues
Total revenues	7,648		13,789
USG
Disaggregation of revenues
Total revenues	43,768	$ 48,890	96,602	$ 104,745
USG | Point in time
Disaggregation of revenues
Total revenues	32,209		72,524
USG | Over time
Disaggregation of revenues
Total revenues	11,559		24,078
USG | United States
Disaggregation of revenues
Total revenues	28,706		63,665
USG | International
Disaggregation of revenues
Total revenues	15,062		32,937
USG | Commercial
Disaggregation of revenues
Total revenues	43,736		95,971
USG | U.S. Government
Disaggregation of revenues
Total revenues	$ 32		$ 631